Many labels and packaging materials are printed using flexography, a printing method that uses a relief plate. Flexographic relief plates may be made from rubber or a photopolymer. Traditional rubber plate precursors may be molded, carved, or ablated with a laser to form the relief. Photopolymer plate precursors are exposed with ultra-violet light through a mask to harden the photopolymer. Then the unexposed polymer is washed out, the plate is dried, then an additional ultra-violet exposure is used to detack or cure any remaining uncured photopolymer. The exposed areas form the relief used to print the image.
The relief is typically 500 um to 1000 um measured from the top of the plate to the floor or non-imaging portion of the plate. For a photopolymer plate the floor is exposed from the back side and may be varied by changing the back side ultra-violet exposure. Both rubber and photopolymer plates are typically mounted to a polyester support. Plates are mounted to printing cylinders or sleeves using a double back compressible tape. Engraved rubber-coated cylinders or sleeves are also used. Unless otherwise specified in the following description, the term plate refers to any form of relief printing member.
In a flexographic printing press, ink is coated onto an Anilox roll and then transferred to the flexographic relief plate. The plate is then pressed against a receiver backed by an impression roller. Receivers may be uncoated paper, coated paper, polymers, glass, ceramics, wood, corrugated board, hard board, or metals. The printed density is dependent upon the Anilox cell volume, the ink, the pressure between the plate and the Anilox roller, the pressure between the plate and the receiver, and the receiver.
To print grey scale images, relief features comprising size-modulated halftone dots or spatial frequency-modulated dots are used. Artistic methods such as line drawings may also be used. The grey scale or tone scale is calibrated by printing test patches with no compensation. The density of each patch is measured and an effective dot area is computed based on the measured density. Then a compensation curve is created to compute the dot area required to obtain a desired printed density.
Flexographic printing has difficulty imaging extremely small dots. Dots between 0% and 5% by area, less than 20 um in diameter, may image extremely dark or not image at all. Typically press operators limit the smallest dot size printed to a minimum of 4-10%, 20-30 um diameter, to avoid these quality issues. The dot gain when printing a 20% dot on plate may result in a density that corresponds approximately to 50% dot area coverage. Flexography has a typical 25-35% dot gain at a 20% input level. The printed density keeps increasing until the 80-90% dot level, at which point density then decreases to the solid density at 100%. This behavior results in a calibration curve that starts at 0%, jumps to a minimum output dot of 4-6%, then a region of image highlights between 4-10%, a region of midtones between 10-30%, a region of shadow details between 40%-85%, and finally solid features are imaged at 100%. The compression of the highlights makes them difficult to control and increases the quantization on the print. The tone scale on press will also depend upon how the plate relief is made and the impression between the plate and the receiver.
Color images are printed using flexography by employing well known color separation techniques wherein each color has its own grey scale image. Calibrating each color and simultaneously controlling every color on press is a challenge. Newer presses with feedback on impression and servo-driven cylinders, along with digitally created plates, have enabled color flexographic printing that rivals offset lithography.
The local relief within a grey scale image will be much lower than the relief between the top of the plate and the floor. For a photopolymer plate a 50% tint will have a local relief depth between dots of 100-200 um. A single 20 um×20 um hole corresponding to a 98.6% halftone at 150 lines per inch will have a depth of 10-30 um.
Recent advances as taught in U.S. Publication No. 2010/0143841 (Stolt et al.) discuss modifying the plate surface by applying a pattern to substantially all image feature sizes of the halftone image data to reduce the transparency of image areas of a mask by a constant amount. The resultant mask can be affixed to a plate precursor to form an intimate contact with, and a gaseous barrier to, the plate precursor. The plate precursor can then be exposed to curing radiation and the mask removed. After processing, the precursor forms a relief plate carrying a relief image that resolves the pattern in the surface of relief features. The print densities of solid features are substantially maintained or increased when the pattern is applied to solid relief features. Among the advantages of using this method are increased dynamic range and more uniform density. Applying a texture pattern to the surface of a flexographic printing plate is performed by advanced screening technology called DigiCap available from Eastman Kodak Company as described at http://graphics.kodak.com/US/en/Product/value_in_print/advancedFlexoScreening/digicapImaging/default.htm.
There are many advantages to encoding data into printed works. One may wish to encode copyright information, additional information about a product, a remote internet address or link, or encrypted data to indicate authenticity or make it more difficult to copy. One common data encoding method is to embed a watermark within the image. U.S. Pat. No. 7,174,031 (Rhoads et al.) list many methods of encoding data in images. In addition it discusses many additional uses for encoded data.
There are many known methods of performing steganography, embedding data, or watermarks, in printed images. There are also many known methods of applying visible data or watermarks in printed images.
U.S. Publication No. 2008/0019559 (Wang et al.) modulates a halftone dot with a screened high frequency pattern. Wang et al. (2008/0019559) modulate each pixel printed on an electrophotographic printer using a different halftone texture. This causes a visible seam between different halftone techniques that creates the visible watermark on the print. Wang et al. (2008/0019559) teaches that “Halftoning techniques are necessary because the physical processes involved are binary in nature or the processes have been restricted to binary operation for reason of cost, speed-memory, or stability in the presence of process fluctuations. Examples of such processes are: most printing presses; ink jet printers; binary cathode ray tube displays; and laser xerography.” [pg. 3 para. 0036]. Adding a high frequency screen to the halftone dot reduces its dot area requiring an additional dot gain table and the printing of a larger dot. Printing a larger dot is a disadvantage in relief printing.
Wang et al. (2008/0019559) also state “Examples of AM-FM halftones include “green-noise” halftones, halftones on space filling curves, and halftones with texture control”, [pg. 3 para. 0042]. Texture control describes the print visibility of high frequency FM noise and sharpness of FM prints verses the visibility of the AM Halftone especially in the highlight areas of the print. The AM-FM Halftoning technique replaces AM screening with FM screening in the highlight areas.
U.S. Publication No. 2010/0060943 (Monga et al.) describes decoding message data embedded in an image print using halftone dot orientation. Bulan et al., “Data Embedding In Hardcopy Images Via Halftone-Dot Orientation Modulation”, Proc of SPIE-IS&T Electronic Imaging, Vol. 6819, (2008), embed data in a print by modulating the orientation of an elliptical halftone dot.
U.S. Pat. No. 7,554,699 (Wang et al.) modulates printed shadow images with hybrid halftone dots consisting of amplitude modulated (AM) dots with frequency modulation (FM). Wang et al. (U.S. Pat. No. 7,554,699) use a hidden bi-level pattern mask (M) to create a combined halftone image (W) using W=(H1∩M)∪(H2∩−M), where H1 is the original AM modulated halftone image; H2 is the image of a FM halftone dot; A∩B is the intersection of A with B; A∪B is the union of A and B. This method results in the following table:
AmplitudeFrequencyWater-ModulatedModulated(M ∩ H1) ∪mark MHalftone H1M ∩ H1Screen H2~M ∩ H2(~M ∩ H2)OffOffOff = H1OffOn = ~H2H1 ∪ ~H2OffOffOff = H1OnOff = ~H2H1 ∪ ~H2OffOnOn = H1OffOn = ~H2H1 ∪ ~H2OffOnOn = H1OnOff = ~H2H1 ∪ ~H2OnOffOn = ~H1OffOff = H2~H1 ∪ H2OnOffOn = ~H1OnOn = H2~H1 ∪ H2OnOnOff = ~H1OffOff = H2~H1 ∪ H2OnOnOff = ~H1OnOn = H2~H1 ∪ H2Areas with no watermark are printed with an inverted FM screen surrounding traditional AM halftone dots. In areas where the watermark is on, the normally AM modulated halftone dot areas are embedded with an FM screen. In areas where the watermark is on the surrounding areas without AM modulated halftone dots are printed as solids.
FM screens use the smallest feature sizes that may print reliably. For offset lithography the smallest feature size may be as small as 10 um by 10 um but more likely printers will use a 20 um by 20 um feature as the smallest available. As feature sizes decrease they are more difficult to control over the length of the print run. For flexography the smallest dot is typically limited to be above 3-5%. At 150 dpi a 3% dot is a 29 um×29 um feature. State of the art flexography is capable of imaging isolated 10 um by 10 um dots, using servo controlled cylinders with servo controlled impression. However state of the art flexography does not image 10 um by 10 um holes in solids such that the calibration curve typically jumps from 85% to 100% making it unobvious to use isolated 10 um by 10 um holes to modulate an AM flexographic dot. If flexography did reproduce 10 um by 10 um holes, and if 10 um by 10 um holes were added to an amplitude modulated halftone dot, for instance a 30% dot, then there would be a large difference in dot area on the print which requires compensation by printing in the background of the AM dot. The large dot gain in relief printing is problematic for the printing of fine stochastic screens and small holes within halftone dots.
Suh et al., “Printer Mechanism-Level Data Information Embedding and Extraction for Halftone Documents—New Results”, Purdue University, embed data in a halftone image by modulating the halftone dot position.
Oztan and Sharma, “Multiplexed Clustered-Dot Halftone Watermarks Using Bi-Directional Phase Modulation and Detection”, Proc. 2010 IEEE 17th International Conference on Image Processing, September 2010, embed watermarks by shifting the phase of the halftone pattern in the area of the watermark. This is another form of moving the centroid of the halftone dots.
U.S. Pat. No. 7,436,977 (Wang et al.) describe using a first stochastic screen in areas outside of a watermark, with a second stochastic screen in areas within the watermark, where the second stochastic screen is multi-partitioned and at least one partition is orthogonal to a partition of the first screen.
U.S. Pat. No. 7,286,685 (Brunk et al.) embed a watermark by modifying the threshold of an error diffusion process when screening an image. Brunk et al. embed the watermark in the error signal of the printed image.
It is known that the surface roughness of the receiver influences the amount of ink transferred. Walker and Fetsko, “A Concept of Ink Transfer”, American Ink Maker, December 1955, show that ink transfer is a function of absorption into paper and splitting of the remaining ink. These characteristics vary as a function of the surface roughness of the receiver as shown by their FIG. 2.
There is a need to be able to embed data in printed images. Ideally the method should not be easily discernable by eye. A method which hides information in halftone dots while minimally modifying the printed halftone pattern are advantaged over techniques that are easily visible, or increase the visible density error, or shift the halftone dots such that they are out of register with subsequent colors.